1. Field of the Invention
The invention relates to differential output driver circuits and to integrated circuits including such output circuits.
2. Description of the Prior Art
A typical differential driver circuit comprises an output driver stage including one or more devices which are driven (alternately) into saturation and produce differential driven outputs on first and second output lines. An impedance is typically connected between the output lines.
FIG. 1 illustrates a prior art output driver circuit 10 which comprises a output driver stage 11 including one or more drive elements which are alternately driven into saturation. The driver stage 11 provides an output signal Y on the first output line 60 by alternatively connecting the output line 60 to a high voltage supply Vdd 16 line and to a low voltage supply line Vss 18 and a complementary driven output Y on line 62 by alternately connecting the output line 62 to the low voltage supply line Vss 18 and to the high voltage supply line Vdd 16 out of phase with the connection of the output line 60.
The output lines 60 and 62 are connected via a nominal resistance of 100.OMEGA. formed by two separate resistors 26 and 28, each having a nominal resistance of 50.OMEGA.. A central tap 24 between the 50.OMEGA. resistors 26 and 28 is supplied to the output stage 11 in order to compare the common mode voltage at the center tap 24 to a common mode reference 30 voltage supplied from an BIAS circuit 12. This feedback is used to control the operation of the output drive stage 11 in order to maintain the common mode voltage at point 24 within a desired range. The output driver stage 11 corrects the output common mode voltage by varying two bias currents from the voltage rails Vdd and Vss, respectively, in opposite directions.
Typically, the various components of the output driver circuit 10 will be contained within an integrated circuit. However, it is notoriously difficult to provide resistive elements on chip using large scale integration techniques with reasonable tolerances. Accordingly, for high precision output drivers, conventional practice is to use external discrete resistors between the differential output line pairs 60, 62 in order to be able to provide an output impedance within desired tolerance ranges.
Accordingly, an aim of the invention is substantially to eliminate or at least to mitigate the problems of prior art differential driver circuits.